In the field of tape recording, both magnetic and optical recording, a constant goal is to increase the total storage capacity on a given media, such as a tape cassette or cartridge. This goal can be achieved in many ways, including the use of longer and/or wider tape.
When the tape length and width are preset, the total capacity may be increased by increasing the number of recorded tracks on the tape, i.e., increasing track density, or by increasing the linear data bit density on each track. Track density is often measured in tracks per inch ("tpi"), while linear bit density is often measured in bits per inch ("bpi").
For many years increasing track density has been a very effective way of increasing the total storage capacity. For example, the popular 3M cartridge, often referred to as a "QIC" cartridge or a "DKC6000" type cartridge, has increased in capacity from 20 MBytes in 1983 to 13 GBytes in 1996. The 20 MBytes capacity was achieved by recording on four tracks on a one-quarter inch wide tape. This therefore gives a track density of 16 tpi. The 13 GByte format employs 144 data tracks on the same tape width. The data track density is therefore 576 tpi. Other types of tape cassettes and cartridges have undergone the same improvements. For example, the small Minicartridge ("DC 2000"- cartridge) and its new companion, the TRAVAN cartridge have over a similar period gone from 8 tracks on a one-quarter inch wide tape, to 76 tracks and soon 109 tracks on an 8 mm wide tape (as used in the TRAVAN cartridge).
In theory, increasing the number of tracks is a very simple way to increase the total storage capacity. In the past, the upper limit to the number of tracks was set by the recording head, the read/right head. With previous technology, it was difficult to design heads which could record and read on vary narrow tracks. However, with the invention of thin film magneto-resistive heads, very narrow recording/read heads can be achieved economically. Therefore, head technology today allows for very narrow tracks, thus making it possible to increase the number of tracks on a given tape width far beyond what was possible just a few years ago.
Unfortunately, another limitation makes it difficult to maximize the number of tracks and use very high track numbers which the new head technology offers, and that relates to the stability of the tape itself as it passes over the head during read/write operations. With very high track densities, it becomes critical that the tape substantially maintains the same position as it passes over the head. To achieve this stability, all tape drives utilize some form of tape guides. Normally, tape guides are placed on each side of the head as shown in FIG. 1. Here, a read/write head 100, a tape 103, tape guides 101, 102, and base plate 104 which mounts the guides, are illustrated. Such tape guides can either be built into the tape cassette itself on the base plate, as it is for the 3M DC6000 cartridge and the DC2000 and TRAVAN cartridges, or can be designed as an integral part of the tape drive as for example as used for small Phillips type cassettes. In either case, the tape guides must be designed so that it can keep the tape in a very stable position as it passes across the head.
To avoid problems of either overwriting previously recorded tracks or not reading in the correct position, the maximum vertical movement, perpendicular to the tape length, allowed for the tape between the tape guides must be only a fraction of the distance between two adjacent tracks. Therefore, as track width, and the distance between adjacent tracks, becomes smaller and smaller, the tape guiding must be improved in corresponding fashion.
Hence, as the head is designed for narrower and narrower track widths, such as based on the new thin film technology, tape guiding has become more and more critical. To keep costs down, most tape guides are designed with fixed flanges. Some high end expensive tape drives utilize guides with one side spring loaded. This is expensive and also requires a fairly high tape tension to work properly. For low-cost small form factor tape drives, such spring loaded tape guides are normally too expensive and impractical, and even may not work in many systems due to low tape tension.
With fixed flanges on the tape guides, the tolerance in tape width and the tolerance in the distance between the upper and lower flanges of a tape guide make it very difficult to design a tape guiding system with the stability required by the new high density, high tpi, thin film heads now available. The tape guides must be designed so that even with a tape having a maximum tape width, or having a maximum tolerance on the tape width, running in a cassette or tape drive where the distance between the tape guides is at its minimum level, the tape can still pass correctly between the guides. Although the tape manufacturers have been able to improve tape slitting very much over recent years, it is still a certain tolerance in the tape width along any given tape. Furthermore, the tape will also expand or contract depending upon humidity and temperature. Therefore, the tape guides must be designed to allow for some tolerance in the tape width.
Coupled with the necessary tolerances of the tape guides themselves, this space allocated for the tolerance in the tape width will unavoidably lead to the tape typically having some play between the guide flanges. Very often this leads to the tape "wandering" between an upper and a lower position during operation as demonstrated in FIG. 2. The total tape wander between guides ("d" in FIG. 2) must be much less than the width of a recorded track, or the distance between two adjacent tracks, in order to avoid writing in an incorrect position, overwriting previously recorded data, or not being able to read previously recorded data. While FIG. 2 exaggerates the tape wander in a normal cartridge, nevertheless the problem of tape wander plays an important role in modern tape drive designs.
In addition to tape "wandering" between the guides, shock and vibrations may also cause the tape to change its position relatively to the head during operations.
To overcome these problems and allow for very narrow tracks, a technology called servo-tracking has been introduced during the 1990's. With this technology, special tracks named servo-tracks are precisely recorded along the tape during manufacturing. The tape drives utilizing such tapes are designed so that they are able to detect the position of the servo-track(s) as the tape passes over the tape, and by means of a voice coil or similar technology, constantly repositions the head so that it stays in the correct position relatively to the tape. This principle is used in the new 13 GB DC 6000 system mentioned earlier. This system has 24 prerecorded servo-tracks in addition to the 144 data tracks. The total track density for this system is thus (144+24).times.4=672 tpi.
Unfortunately, while such servq systems make it possible to design tape drives with very high track density, it also adds significantly to the cost and complexity of the tape drive and the tape cartridge itself. Low cost tape drives, often with a small form factor, cannot easily utilize such complex and costly servo-designs. For these drives, attempts have been made to improve the number of tracks in a tape cartridge by constantly reducing the tolerance of the tape width and the play between the tape and tape guides. However, with tighter and tighter tolerances, the cost of improving the guiding is increasing significantly.
Therefore, it is advantageous to provide a method which makes it possible to monitor the variations in tape position, and correct for these variations, without adding significantly to the cost or complexity of the tape drive or the tape cartridge. Such a method is described for example in U.S. Pat. No. 4,639,796. This patent teaches the method of utilizing light to monitor either one or both edges of the tape by placing a transmitter along one side of a tape edge and a receiver on the opposite side of the tape edge. The variation in tape position will result in a varying light signal detected by the receiver. The signal is converted to a corresponding electrical signal which can be used to either make the necessary corrections in the head position or in the cartridge position so that the relative position between the tape and the head is stable along a track.
While the principle of U.S. Pat. No. 4,639,796 will work adequately in many systems, small tape cassettes and drive systems having a small form factor may make it difficult or sometimes impossible to position the light transmitter and receiver correctly on opposite sides of the tape edge. The purpose of this invention is therefore to overcome this problem by a new method which makes it simple to utilize the principle of tape edge following using light, even for physically very small tape cassette systems.